Method for secondary-actuator failure detection and recovery in a dual-stage actuator disk drive

ABSTRACT

A method for secondary-actuator failure-detection and recovery in a dual-stage actuator disk drive includes running a calibration test by the servo control processor and measuring the position of the secondary actuator relative to its neutral position in response to the calibration test. The secondary-actuator failure detection and calibration test can be performed on a regular schedule or at selected times, such as at disk drive start-up. With the primary actuator biased at a test location, such as a crash stop or a load/unload ramp, the servo control processor generates a test signal to the secondary actuator and receives a relative-position signal (RPS) from the relative-position sensor in response to the test signal. The test comprises two measurements: a measurement of the secondary actuator static characteristics, and a measurement of the secondary actuator dynamic characteristics.

RELATED APPLICATION

This application is related to concurrently-filed co-pending applicationSer. No. ______ and titled “DUAL-STAGE ACTUATOR DISK DRIVE WITHSECONDARY ACTUATOR FAILURE DETECTION AND RECOVERY USINGRELATIVE-POSITION SIGNAL”.

This application is also related to previously-filed pending applicationSer. No. 10/997,153 filed Nov. 24, 2004 and titled “DISK DRIVE WITH ADUAL-STAGE ACTUATOR AND FAILURE DETECTION AND RECOVERY SYSTEM FOR THESECONDARY ACTUATOR”.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to magnetic recording hard disk drives,and more particularly to a disk drive with a dual-stage actuator forpositioning the read/write heads.

2. Description of the Related Art

Magnetic recording hard disk drives with dual-stage actuators forpositioning the read/write heads on the disks have been proposed. Arotary voice-coil-motor (VCM) is typically the primary actuator, withthe secondary actuator attached to the VCM and the read/write headsattached to the secondary actuator. A servo control system receivesservo positioning information read by the read/write heads from the datatracks and generates control signals to the primary and secondaryactuators to maintain the heads on track and move them to the desiredtracks for reading and writing of data. As in conventional single-stageactuator disk drives, each read/write head is attached to the end of ahead carrier or air-bearing slider that rides on a cushion or bearing ofair above the rotating disk. The slider is attached to a relativelyflexible suspension that permits the slider to “pitch” and “roll” on theair bearing, with the suspension being attached to the end of the VCMactuator arm. The secondary actuator is typically a piezoelectric orelectrostatic milliactuator or microactuator located on the VCM actuatorarm for driving the suspension, or on the suspension between thesuspension and the slider for driving the slider, or on the slider fordriving just the read/write head.

The conventional servo control system for a typical dual-stage actuatordisk drive uses a controller designed to assure stability of the VCMwith adequate stability margins as if it were to operate without thesecondary actuator. Then the controller for the secondary actuator isdesigned to achieve the desired combined dual-stage bandwidth. Thesecondary actuator control loop and the combined dual-stage control loopare also designed to ensure adequate stability separately and jointlywith the other control loops. This type of servo control system issatisfactory for limited increases in the bandwidth above what isachievable with only the VCM.

In co-pending application Ser. No. 10/802,601 filed Mar. 16, 2004,titled “MAGNETIC RECORDING DISK DRIVE WITH DUAL-STAGE ACTUATOR ANDCONTROL SYSTEM WITH MULTIPLE CONTROLLERS”, and assigned to the sameassignee as this application, a dual-stage actuator disk drive isdescribed that operates with an improved servo control system that hastwo controllers. One controller is a dual-stage controller thatsimultaneously generates a primary actuator control signal and asecondary actuator control signal, and uses a degraded-stability primaryactuator controller design with relatively high low-frequency open-loopgain and a secondary actuator controller design that provides stabilityand high mid-frequency to high-frequency open-loop gain resulting inincreased bandwidth. The other controller is a single-stage controllerthat generates only a primary actuator control signal and uses a stableVCM-only controller design. If a potential failure of the secondaryactuator is detected, the servo control system selects the single-stagecontroller.

In dual-stage actuator disk drives with either the conventional servocontrol system or the control system of the co-pending application, afailure of the secondary actuator will result in reduced performance andmay lead to loss of data and/or failure of the disk drive. In therelated co-pending application Ser. No. 10/997,153 filed Nov. 24, 2004,titled “DISK DRIVE WITH A DUAL-STAGE ACTUATOR AND FAILURE DETECTION ANDRECOVERY SYSTEM FOR THE SECONDARY ACTUATOR”, and assigned to the sameassignee as this application, a secondary-actuator failure detectiontest is performed by generating a test signal to the secondary actuatorand measuring a calibration signal from the read head as the read headdetects test blocks in special calibration tracks located on the disk.If the calibration signal indicates only reduced performance of thesecondary actuator from which failure is recoverable, the controllerparameters are adjusted. The writing of the special calibration trackscontaining the test blocks increases the time and cost of theservowriting process.

What is needed is a dual-stage actuator disk drive that can detectactual failure of the secondary actuator and modify the controllerparameters if the failure is recoverable, but without the need forspecial calibration tracks on the disk.

SUMMARY OF THE INVENTION

This invention is a method for secondary-actuator failure-detection andrecovery in a dual-stage actuator disk drive where a calibration testrun is by the servo control processor and the position of the secondaryactuator is measured relative to its neutral position in response to thecalibration test. The secondary-actuator failure detection andcalibration test can be performed on a regular schedule or at selectedtimes, such as at disk drive start-up. With the primary actuator at aspecific test location, such as an actuator crash stop or theload/unload ramp, the servo control processor generates a test signal tothe secondary actuator and receives a relative-position signal (RPS)from the relative-position sensor in response to the test signal.

The test comprises two measurements: a measurement of the secondaryactuator static characteristics, and a measurement of the secondaryactuator dynamic characteristics. In both measurements, the RPS is usedto measure the secondary actuator movement relative to its neutralposition.

The static characteristics measurement is a calculation of the secondaryactuator “stroke”, i.e., the amount of secondary actuator movement as afunction of voltage input to the secondary actuator, and a comparison ofthe calculated stroke to a predetermined range of acceptable strokevalues. With the primary actuator biased against the crash stop orload/unload ramp, a first test signal is applied to the secondaryactuator and the RPS is used to calculate the stroke. The calculatedstroke values are averaged over several disk rotations. If thecalculated stroke is outside the acceptable range, then this is anindicator that the secondary actuator has likely failed.

If the calculated stroke is within the acceptable range, then thedynamic characteristics measurement is made. This measurement isessentially a measurement of the plant frequency response of thesecondary actuator. With the primary actuator biased against the crashstop or load/unload ramp, a second test signal is applied to thesecondary actuator and the RPS is recorded. The second test signal forthe dynamic characteristics measurement is a series of test signals,each a sinusoidal signal at a constant frequency. The RPS is detectedduring the application of the constant frequency test signal, and theresulting gain and phase of the response are recorded along with thecorresponding frequency. This is repeated for each frequency in theseries of test signals. This enables the plant frequency response of thesecondary actuator to be measured. If the response is significantlydifferent from the expected response, it is virtually assured that thesecondary actuator has failed. If the measured frequency response showsminor changes, such as a minor increase or decrease in the gain, or aminor shift in the frequency at which the maximum gain occurs, thecontroller parameters are adjusted or re-optimized. This re-optimizationchanges the values of the controller parameters in the memory accessibleby the servo control processor. The parameters that can be changedinclude parameters that affect bandwidth or stability margins, notchingof particular frequencies such as the secondary actuator resonantfrequency, active damping of the secondary actuator resonance, or otherperformance, robustness, or stability metrics.

The invention is applicable to both the dual-stage actuator disk drivewith a servo control system having a conventional dual-stage controller,and the dual-stage actuator disk drive according to the previously-citedco-pending application Ser. No. 10/802,601 that has both a dual-stagecontroller and a selectable single-stage controller.

In the dual-stage actuator disk drive according to the co-pendingapplication Ser. No. 10/802,601, a potential failure of the secondaryactuator is detected either by providing a model of the dynamic responseof the primary and secondary actuators and comparing the modeledhead-position with the measured head-position, or by measuring therelative position of the secondary actuator with the relative-positionsensor and comparing the relative position to a modeled position of thesecondary actuator. Upon detection of a potential failure of thesecondary actuator, the single-stage controller is selected, the primaryactuator is moved to the test location, and the secondary-actuatorfailure detection and calibration test is run. If the secondary actuatorpasses both the static characteristics measurement test and the dynamiccharacteristics measurement test, then the dual-stage controller isre-selected. If the measured frequency response shows minor changes fromthe optimized frequency response, the controller parameters are adjustedor re-optimized prior to re-selection of the dual-stage controller.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a prior art single-stage actuator diskdrive.

FIG. 2 is an open-loop frequency response for a prior art disk drivewith only a single-stage actuator, typically a voice-coil-motor (VCM).

FIG. 3 is a view of a secondary actuator usable with the above-describedprior art disk drive.

FIG. 3A is an exploded view illustrating the relationship and operationof a prior art rotary electrostatic microactuator, a slider and asuspension.

FIG. 4 is a comparison of an open-loop frequency response for a priorart disk drive with a dual-stage actuator with a stable VCM controllerdesign and the open-loop frequency response of FIG. 2.

FIG. 5 is a block diagram of a disk drive with a dual-stage actuator andcontrol system according to this invention.

FIG. 6 is a comparison of the open-loop frequency response for thedual-stage controller with a degraded-stability VCM controller designaccording to this invention and the open-loop frequency response of FIG.2.

FIG. 7 shows the gain of the VCM open-loop frequency response for astable single-stage VCM controller, the gain improvement with theaddition of a secondary actuator, and the further gain improvement withthe use of an increased-gain degraded-stability VCM controller and asecondary actuator.

FIG. 8 is a schematic structure of the control system of this invention.

FIG. 9 is a flow chart for the operation of the disk drive of thisinvention.

FIG. 10 is a schematic of rotary electrostatic microactuator, itsassociated driving circuitry, and the relative-position sensor thatprovides the RPS used in the secondary-actuator failure detection andcalibration test of the present invention.

FIG. 11 is a flow chart of the details of the secondary actuator staticcharacteristics measurement in this invention.

FIG. 12 is a flow chart of the details of the secondary actuator dynamiccharacteristics measurement in this invention.

FIG. 13 shows a typical test signal at a fixed frequency and a RPSresponse for the measurement of the secondary actuator dynamiccharacteristics.

FIG. 14 is an example of the plant frequency response for a typicalmicro-electro-mechanical system (MEMS) type microactuator locatedbetween the suspension and the slider.

DETAILED DESCRIPTION OF THE INVENTION

Prior Art

FIG. 1 is a block diagram of a conventional single-stage actuator diskdrive that uses servo positioning information located inangularly-spaced servo sectors for positioning the read/write heads. Thedisk drive, designated generally as 100, includes data recording disk104, a voice coil motor (VCM) 110 as the primary and only actuator, aninner-diameter (ID) crash stop 102 and an outer-diameter (OD) crash stop103 for the VCM 110, an actuator arm 106, a suspension 107, a headcarrier or air-bearing slider 108, a data recording transducer 109 (alsocalled a head, recording head or read/write head), read/writeelectronics 113, servo electronics 112, and servo control processor 115.If the disk drive is a “load/unload” type of disk drive wherein theslider 108 is lifted off or “unloaded” from the disk when the disk driveis not operating, then a load/unload ramp 105 is provided for supportingthe slider 108. The VCM 110 moves near or beyond the perimeter of thedisk and the suspension rides up the ramp 105 to unload the slider 108from the disk 104.

The recording head 109 may be an inductive read/write head or acombination of an inductive write head with a magnetoresistive read headand is located on the trailing end of slider 108. Slider 108 issupported on the actuator arm 106 by a suspension 107 that enables theslider to “pitch” and “roll” on an air-bearing generated by the rotatingdisk 104. Typically, there are multiple disks stacked on a hub that isrotated by a disk motor, with a separate slider and recording headassociated with each surface of each disk.

Data recording disk 104 has a center of rotation 111 and is rotated indirection 130. The disk 104 has a magnetic recording layer withradially-spaced concentric data tracks, one of which is shown as track118. The disk drive in FIG. 1 is illustrated as a zone-bit-recording(ZBR) disk drive because the data tracks are grouped radially into anumber of annular data zones or bands, three of which are shown as bands151, 152 and 153, but the invention is fully applicable to a disk drivethat does not use ZBR, in which case the disk drive would have only asingle data band. The range of rotation of the VCM is between the IDcrash stop 102 and the OD crash stop 103, which define the ID and OD ofthe annular region where data can be written. Each data track has areference index 121 indicating the start of track. Within each band, thetracks are also circumferentially divided into a number of data sectors154 where user data is stored. If the disk drive has multiple heads,then the set of tracks which are at the same radius on all diskrecording layers is referred to as a “cylinder”.

Each data track also includes a plurality of circumferentially orangularly-spaced servo sectors. The servo sectors in each track arealigned circumferentially with the servo sectors in the other tracks sothat they extend across the tracks in a generally radial direction, asrepresented by radially-directed servo sections 120. The servopositioning information in each servo sector includes a servo timingmark (STM), a track identification (TID) code, and a portion of apattern of magnetized blocks or high-frequency bursts that are decodedto provide a head position error signal (PES).

The servo positioning information in the servo sectors is read by theread/write electronics 113 and signals are input to the servoelectronics 112. The servo electronics 112 provides digital signals toservo control processor 115. The servo control processor 115 provides anoutput 191 to VCM driver 192 that controls current to the VCM 110 toposition the head 109.

Within the servo electronics 112, the STM decoder 160 receives a clockeddata stream from the read/write electronics 113. Once an STM has beendetected, an STM found signal is generated. The STM found signal is usedto adjust timing circuit 170, which controls the operating sequence forthe remainder of the servo sector.

After detection of an STM, the track identification (TID) decoder 180receives timing information from timing circuit 170, reads the clockeddata stream, which is typically Gray-code encoded, and then passes thedecoded TID information to servo control processor 115. Subsequently,the PES decoder 190 (also called the servo demodulator) captures theposition information from read/write electronics 113 and passes aposition error signal (PES) to servo control processor 115.

The servo control processor 115 includes a microprocessor 117 that usesthe PES as input to a control algorithm to generate the control signal191 to VCM driver 192. The control algorithm recalls from memory a“controller” 116, which is a set of parameters based on the static anddynamic characteristics of the “plant” being controlled, i.e., the VCM110. The controller 116 is a “single-stage” controller because the diskdrive being described has only a primary actuator, i.e., VCM 110, andservo control processor 115 provides only a single output, i.e., signal191 to VCM driver 192. The control algorithm is essentially a matrixmultiplication algorithm, and the controller parameters are coefficientsused in the multiplication and stored in memory accessible by themicroprocessor 117.

The method of designing the controller 116 is well-known in the digitalservo control and disk drive servo control literature. The controllercan be designed in the frequency domain to achieve the desired open-loopinput-output frequency response of the VCM 110. The input-outputbehavior of a dynamic system at any frequency can generally be expressedby two parameters, the gain (G) and the phase (φ) representing theamount of attenuation/magnification and phase-shift, respectively. Thegain and phase of a dynamic system represent the frequency response ofthe system and can be generated by experiment. In disk drivesingle-stage servo control systems the controller 116 must be a stabledesign. FIG. 2 is an example of an open-loop frequency response 210 fora disk drive with only a single-stage actuator, i.e., VCM 110. Thesingle-stage controller 116 for this system assures stability. Forexample, at the gain zero-crossover, the phase margin is about 30degrees and at −180 degree phase the gain margin is about 5 dB. Also, ata natural resonance near 4 kHz, the phase is near zero such that thesystem is phase stable.

FIG. 3 shows one example of a secondary actuator usable with theabove-described disk drive wherein the secondary actuator is a rotarymicroactuator 200 located between the slider 108 and the suspension 107.Rotation of the microactuator 200 as represented by arrow 202 causesrotation of the slider 108 and thus movement of head 109 in thecross-track direction. The microactuator 200 maintains the head 109on-track, while the VCM 110 moves the slider 108 (and thus head 109)across the tracks. Other types of secondary actuators are alsowell-known, such as piezoelectric-based actuators. Also, the secondaryactuator may be located on the suspension or actuator arm to move asuspension or arm-section relative to the fixed actuator arm, as in U.S.Pat. No. 5,936,805, or between the slider and a slider-segment to movethe head relative to the slider, as in U.S. Pat. No. 6,611,399.

FIG. 3A is an exploded view of microactuator 200, the disk drivesuspension 107, and the air-bearing slider 108. The slider 108 has anair-bearing surface 32 that faces the disk of the disk drive and atrailing surface 34 that supports the read/write head 109. Themicroactuator 200 depicted in FIG. 3A is a rotary electrostaticmicroactuator as described in detail in U.S. Pat. No. 5,959,808 and L.S. Fan et al., “Electrostatic Microactuator and Design Considerationsfor HDD Applications”, IEEE Transactions on Magnetics, Vol. 35, No. 2,March 1999, pp. 1000-1005. The rotary electrostatic microactuator 200comprises fixed and movable portions on an insulating substrate 51,which is typically a silicon base with an insulating surface layer, suchas a layer of polyimide, silicon oxide or silicon nitride. The substrate51 is mounted to the end of the suspension 107. The fixed portion of themicroactuator 200 includes a central post 52 fixed to substrate 51 andtwo sets of stationary electrodes 53, 53′ also fixed to substrate 51.The movable rotary portion of the microactuator 200 includes a supportframe 54, a plurality of flexible members or springs 55 connecting theframe 54 to the fixed post 52, and a plurality of movable electrodesdepicted as two sets 56, 56′ attached to frame 54. The sets of movableelectrodes 56, 56′ are interleaved with the sets of fixed electrodes 53,53′, respectively. The frame 54 may optionally support a platform towhich the slider 168 is bonded. All of the elements attached to thesubstrate 51 are formed by lithographically patterning the substrate 51and depositing electrically conductive material, such as nickel orcopper.

A voltage applied to stationary electrodes 53 will apply anelectrostatic attractive force between stationary electrodes 53 andmovable electrodes 56, tending to cause the frame 54 to rotatecounterclockwise. A voltage applied to stationary electrodes 53′ willapply an electrostatic attractive force between stationary electrodes53′ and movable electrodes 56′, tending to cause the frame 54 to rotateclockwise. Thus a first set of stationary electrodes 53 and movableelectrodes 56 are associated with counterclockwise rotation and a secondset of stationary electrodes 53′ and movable electrodes 56′ areassociated with clockwise rotation. The frame 54 rotates (as depicted byarrows 202) due to the flexibility of the springs 55 and thus moves theread/write head 109 to maintain its position on a data track on thedisk.

The conventional control system for a disk drive with a dual-stageactuator is similar to that described above except that there is asecond output from the servo control processor that is sent to thedriver for the secondary actuator, and the controller is a dual-stagecontroller. In the dual-stage control system, the VCM or primaryactuator is typically responsible for large-amplitude, low-frequencymotion and the microactuator or secondary actuator is typicallyresponsible for small-amplitude, high-frequency motion. The design of adual-stage controller for a hard disk drive dual-stage servo controlsystems is well-known, as described for example, in Y. Lou et al.,“Dual-Stage Servo With On-Slider PZT Microactuator for Hard DiskDrives”, IEEE Transactions on Magnetics, Vol. 38, No. 5, September 2002,pp. 2183-2185; and T. Semba et al., “Dual-stage servo controller for HDDusing MEMS microactuator”, IEEE Transactions on Magnetics, Vol. 35,September 1999, pp. 2271-2273. Generally, the design of a dual-stagecontroller starts with the VCM controller, typically with a design thatis very similar to a single-stage VCM controller, such as the design forthe VCM controller whose open-loop frequency response 210 is shown inFIG. 2. In particular, the stability of the VCM is assured with adequaterobustness or stability margins as if it were to operate without thesecondary actuator. Then the controller for the secondary actuator isdesigned to achieve the desired combined dual-stage bandwidth. Thesecondary actuator control loop and the combined dual-stage control loopare also designed to ensure adequate stability separately and jointlywith the other control loops. This process is satisfactory for limitedincreases in the bandwidth above what is achievable with only the VCM.

An example of an open-loop frequency response for a dual-stage hard diskdrive with a conventional stable VCM controller design is shown asfrequency response 220 in FIG. 4 and compared with the frequencyresponse 210, which is also shown FIG. 2. The response 220 is similar toresponse 210 at low frequencies, but has higher gain in themid-frequency range, and a higher bandwidth. For disk drives in generaland for the frequency responses described herein, the low frequencyrange is generally meant to be below approximately 300 Hz, the midfrequency range is generally meant to be approximately 300 Hz to 2 kHz,and the high frequency range is generally meant to be aboveapproximately 2 kHz. However, the boundaries between what are consideredlow, mid, and high frequency ranges are more accurately linked to wherethe microactuator begins to dominate the overall frequency response andthe open loop bandwidth (0 dB crossover) achievable with a single-stageactuator. This is strongly related to the physical size of the diskdrive.

The Invention

In a dual-stage actuator disk drive as described above, failure of thesecondary actuator will result in reduced performance and may lead toloss of data and/or failure of the disk drive. Thus it is important tobe able to detect actual failure of the secondary actuator andre-optimize the controller parameters if the failure is recoverable.

This invention is a dual-stage actuator disk drive that uses asecondary-actuator failure-detection and calibration test run by theservo control processor and a relative-position sensor for measuring theposition of the secondary actuator relative to its neutral position inresponse to the calibration test. The secondary-actuator failuredetection and calibration test can be performed on a regular schedule orat selected times, such as at disk drive start-up. With the primaryactuator biased against a crash stop or load/unload ramp, the servocontrol processor generates a test signal to the secondary actuator andreceives a relative-position signal (RPS) from the relative-positionsensor in response to the test signal.

The test comprises two measurements: a measurement of the secondaryactuator static characteristics, and a measurement of the secondaryactuator dynamic characteristics. In both measurements, the RPS is usedto measure the secondary actuator movement relative to its neutralposition.

The static characteristics measurement is a calculation of the secondaryactuator “stroke”, i.e., the amount of secondary actuator movement as afunction of voltage input to the secondary actuator, and a comparison ofthe calculated stroke to a predetermined range of acceptable strokevalues. With the primary actuator biased against the crash stop orload/unload ramp, a first test signal is applied to the secondaryactuator and the RPS is used to calculate the stroke. The calculatedstroke values are averaged over several disk rotations. If thecalculated stroke is outside the acceptable range, then this is anindicator that the secondary actuator has likely failed.

If the calculated stroke is within the acceptable range, then thedynamic characteristics measurement is made. This measurement isessentially a measurement of the plant frequency response of thesecondary actuator. With the primary actuator biased against the crashstop or load/unload ramp, a second test signal is applied to thesecondary actuator and the RPS is recorded. The second test signal forthe dynamic characteristics measurement is a series of test signals,each a sinusoidal signal at a constant frequency. The RPS is detectedduring the application of the constant frequency test signal, and theresulting gain and phase of the response are recorded along with thecorresponding frequency. This is repeated for each frequency in theseries of test signals. This enables the plant frequency response of thesecondary actuator to be measured. If the response is significantlydifferent from the expected response, it is virtually assured that thesecondary actuator has failed. If the measured frequency response showsminor changes, such as a minor increase or decrease in the gain, or aminor shift in the frequency at which the maximum gain occurs, thecontroller parameters are adjusted or re-optimized. This re-optimizationchanges the values of the controller parameters in the memory accessibleby the servo control processor. The parameters that can be changedinclude parameters that affect bandwidth or stability margins, notchingof particular frequencies such as the secondary actuator resonantfrequency, active damping of the secondary actuator resonance, or otherperformance, robustness, or stability metrics.

While the invention is fully applicable to a dual-stage actuator diskdrive with a servo control system having a conventional dual-stagecontroller as described above, the invention will be described in detailbelow as implemented in the dual-stage actuator disk drive with theimproved servo control system of the previously-cited co-pendingapplication Ser. No. 10/802,601.

FIG. 5 is a block diagram of the control system of the present inventionfor a dual-stage hard disk drive. The head 109 reads the servo patternfrom the disk, the read/write electronics 113 processes the signal fromthe head 109, and the servo electronics 112 generates the PES from thesignals from read/write electronics 113, all as described in the priorart.

The servo control processor 400 receives the PES from servo electronics112, and provides a primary control signal 191 to VCM driver 192 and asecondary control signal 229 to microactuator driver 230. The servocontrol processor includes a microprocessor 117 and uses a dual-stagecontroller 410 to generate control signals 191, 229. The dual-stagecontroller 410 incorporates a degraded-stability VCM controller withrelatively high low-frequency open-loop gain, and a secondary actuatorcontroller providing stability to the dual-stage controller and highmid-to-high-frequency open-loop gain, resulting in increased bandwidth.However, if the microactuator 200 fails while the disk drive is underthe control of dual-stage controller 410, then VCM 110 will becomeunstable. If the microactuator 200 fails then the servo controlprocessor 400 switches to use of a single-stage stable controller 420and generates only a primary control signal 191 to VCM driver 192. Thesingle-stage controller 420 can be a VCM controller based on thefrequency response 210 (FIG. 2) or any VCM controller that is stablewithout the microactuator 200. This stable VCM controller 420 will mostlikely have decreased performance, but will prevent catastrophic failureof the disk drive that would result in loss of data.

In the preferred embodiment, the detection of potential failure ofmicroactuator 200 is by a microactuator relative-position sensor 240.The sensor 240 measures the displacement of microactuator 200 relativeto its neutral position (and thus to VCM 110) and provides arelative-position signal (RPS) to servo control processor 400. If thesecondary actuator is an electrostatic microactuator, then sensor 240can be a capacitance sensing circuit, as described in M. T. White and T.Hirano, “Use of the Relative Position Signal for Microactuators in HardDisk Drives”, Proceedings of the American Control Conference, Denver,Colo., Jun. 4-6, 2003, pp. 2535-2540.

FIG. 6 is the open-loop frequency response 412 for the dual-stagecontroller 410 with a degraded-stability VCM controller design comparedwith the frequency response 210 for the single-stage stable controller420. As frequency response 412 shows, the low frequency gain may beincreased by relaxing the stability requirements of the VCM, but at theexpense of robustness. The resulting open-loop frequency response of thedual-stage system 412 has increased gain over a wider frequency comparedto the open-loop frequency response of the dual-stage system 220 shownin FIG. 4. This will result in better disturbance rejection andperformance. The frequency response of the dual-stage system 412 hasgain and phase margins that are comparable to the frequency response ofthe single-stage system 210 in FIG. 2. The phase margin near 2.8 kHz isabout 30 degrees and the gain margin near 3.8 kHz is about 5 dB.

FIG. 7 is the gain portion of three frequency responses. Solid line 210represents the gain of the VCM open-loop frequency response for a stablesingle-stage VCM controller. Dotted line 220 represents the improvementto response 210 with the addition of the secondary actuator(microactuator 200) and is the typical shape for a conventionaldual-stage controller. It has increased bandwidth and increased gain inthe mid-frequency range. This will result in improved disturbancerejection and faster response at these frequencies. However, because thelow-frequency gain is still determined by the single-stage VCMcontroller, there is no improvement at low frequency. Cross-hatched line412 represents the further improvement with the use of an increased-gaindegraded-stability VCM controller and the secondary actuator. Thisresponse also has increased low-frequency gain, and is comparable inshape to the response for the VCM-only design, but shifted higher infrequency. However, increasing the low-frequency gain will also decreasethe phase margin for the VCM controller, potentially to the point ofinstability of the VCM. The secondary actuator controller is thendesigned to make the combined system stable, as well as increasing themid-frequency to high-frequency gain. Using the secondary actuator toensure the stability of the combined system typically takessignificantly less stroke than using the secondary actuator to increasethe low-frequency gain, and is therefore a more efficient use of thelimited secondary actuator stroke to achieve high bandwidth withadequate disturbance rejection.

With the dual-stage controller having the characteristics represented byline 412 in FIG. 7, failure of the secondary actuator results in anunstable system. This could lead to inoperability of the hard diskdrive, or even failure with loss of data. This is avoided by detecting apotential failure of the secondary actuator and switching to a stableVCM-only controller. The potential failure of the secondary actuator isdetected either by providing a model of the dynamic response of theprimary and secondary actuators and comparing the modeled head-positionwith the measured head-position, or by measuring the position of thesecondary actuator relative to the primary actuator with therelative-position sensor 240 and comparing the relative position to amodeled position of the secondary actuator. In the present invention,after a potential failure of the secondary actuator has been detectedand switching to the VCM-only controller has occurred, thesecondary-actuator failure-detection and calibration test is run by theservo control processor.

A schematic structure of the control system of the present invention isshown in FIG. 8. C_(MACT) and P_(MACT) represent the microactuatorcontroller and plant, respectively, and C_(VCM) and P_(VCM) representthe VCM controller and plant, respectively. The controllers C_(MACT) andC_(VCM) together represent the dual-stage controller 410. Themicroprocessor 117 in servo control processor 400 (FIG. 5) runs thecontrol algorithm using the parameters of controllers C_(MACT) andC_(VCM) and generates control signals u_(MACT) and u_(VCM) (229 and 191,respectively, in FIG. 5). The control system includes a model 430 of themicroactuator plant and a model 440 of the VCM plant. These models maybe determined from frequency response measurements of the microactuatorand VCM, finite element models (FEM), or other well-known systemidentification techniques.

FIG. 8 shows two methods for determining potential failure of themicroactuator 200. In the preferred method the calculated microactuatorcontrol signal u_(MACT) is input to the microactuator model 430 and theestimated microactuator position y_(MACT(EST)) from the model iscompared to the RPS from sensor 240 at junction 450. In an alternative“PES-based” method the calculated microactuator control signal u_(MACT)is input to the microactuator model 430 and the calculated VCM controlsignal u_(VCM) is input to the VCM model 440. The modeled expected orestimated output y_(EST) is then compared with the measured outputy_(MEAS) at junction 460.

FIG. 9 is a flow chart for the operation of the disk drive of thepresent invention. The flow chart portion 500 describes the method fordetecting potential failure of the secondary actuator and switching tothe VCM-only stable controller if potential failure is detected, andflow chart portion 600 describes the method for detecting actual failureof the secondary actuator and possible re-optimization of the controllerparameters to enable switching back to the dual-stage controller havinga degraded-stability VCM controller.

Referring first to portion 500, the control system starts (block 501)and continues to operate using the dual-stage controller 410 with thedegraded-stability VCM controller (block 505). In block 510, theposition of the head is measured (y_(MEAS)) if the PES-based method isused, or the relative position of microactuator 200 is measured (RPS) ifthe relative position sensing method is used. In block 515 the expectedor estimated head position y_(EST) is calculated from the models 430,440 if the PES-based method is used, and the expected or estimatedrelative position (y_(MACT(EST))) is calculated from microactuator model430 if the relative position sensing method is used. The difference(DIFF) is then tested to see if it is within pre-determined bounds(block 520). If yes, the control continues (block 505).

If DIFF is outside the bounds, this indicates potential failure of themicroactuator 200. Once a potential failure of the secondary actuatorhas been detected the servo control processor 400 recalls the stable VCMcontroller 420 (FIG. 5) from memory (block 525). The processor sends alocation signal to the VCM 110 to move the head to the test location,where further movement of the VCM is substantially limited. In thepreferred embodiment, the test location is a crash stop, so the VCM 110is then biased to maintain its position at the inner diameter with theID crash stop 102 compressed (block 530). In this way, the position ofthe VCM is essentially fixed. Because the VCM is biased against thecrash stop, the effects of other sources of error are virtuallyeliminated. This is a safe and accurate way to determine the motion ofthe microactuator in a condition that is virtually isolated from theVCM. The operation then moves to the steps described in flow chartportion 600. In an alternative embodiment, if the disk drive is aload/unload disk drive, the processor sends a signal to VCM 110 to moveto the ramp 105 and the VCM 110 is then biased so that it maintains itsposition at the ramp 105. The following description of the invention ismade with a crash stop being the test location, but the invention isfully applicable if the load/unload ramp is the test location.

Referring to portion 600, the secondary-actuator static characteristicsmeasurement is performed (block 605). With the position of the VCM 110biased against the ID crash stop, a test signal is applied to themicroactuator 200 to calculate its stroke. At the check point (block610) if the stroke is two low or too high, as compared to predeterminedacceptable stroke values, then the secondary actuator has failed. Thesecondary actuator is disabled by selecting the VCM-only controller andan error is posted to the disk drive system (block 615). If thecalculated stroke meets the acceptable criteria, then thesecondary-actuator dynamic characteristics measurement is started (block620).

The secondary actuator dynamic characteristics measurement measures thesecondary actuator plant frequency response. At the check point (block625) if the measured response is significantly different from theexpected result, then the secondary actuator is disabled by selectingthe VCM-only controller and an error is posted to the disk drive system(block 630). If the measured response shows only minor changes, such asa minor gain increase or decrease, or a minor shift in the peakfrequency, then the secondary actuator can be recovered from potentialfailure (block 635). The controller parameters are then adjusted orre-optimized (block 640). The dual-stage controller with thedegraded-stability VCM controller and the new secondary actuatorcontroller parameters is re-selected (block 505) and operationcontinues. As an additional feature, a counter counts the number oftimes the controller parameters are re-optimized, which indicates thenumber of times a failure of the secondary actuator has been detectedbut both the static and dynamic characteristics measurements have beensuccessful. If this count exceeds a certain threshold within a certaintime period, indicating that failures are becoming too frequent, thensubsequent modification of the controller parameters is terminated, anerror is posted and the secondary actuator is disabled (block 630).

FIG. 10 is a schematic showing the rotary electrostatic microactuator200 (FIG. 3A), its associated driving circuitry, and therelative-position sensor 240 that provides the RPS used in thesecondary-actuator failure detection and calibration test of the presentinvention. Because the output force of an electrostatic microactuator isproportional to the square of the voltage difference between thestationary and movable electrodes (53 and 56; 53′ and 56′), it isdesirable to linearize the voltage-vs.-force relationship. FIG. 10 showsthe common differential driving method. In this method, an analogcontrol voltage x is the input to the system. This voltage is processedin two ways, and applied to the two input terminals of themicroactuator. In one path, a fixed bias voltage of A is added, followedby fixed gain amplification G by a high voltage amplifier, resulting ina voltage G*(A+x), which is connected to the microactuator's V1 terminalfor the stationary electrodes 53. In another path, the analog inputsignal x is inverted, and the same fixed bias voltage A is added, andamplified by the same fixed gain G, resulting in a voltage G*(A−x),which is connected to the microactuator's V2 terminal for the stationaryelectrodes 53′. Since the voltage V1 generates counter-clockwise torqueand the voltage V2 generates clockwise torque, the net torque will bethe difference of these two torques, which is proportional to(G*(A+x))²−(G*(A−x))²=4G ² Ax.The result is that the torque is linear to the input control voltage x.

The relative-position of the microactuator 200, i.e. the position ofrotatable frame 54 relative to its neutral position, can be determinedby modeling the electrostatic microactuator as two variable capacitors.The capacitance between electrodes 53 and 56 is equal to the capacitancebetween electrodes 53′ and 56′ when the frame 54 is at its neutralposition. When the frame 54 moves from its neutral position, one of thecapacitances increases and the other capacitance decreases. A sensingsignal V_(s)*sin(w_(s)*t) is added to one side of the drive signalG*(A+x) and subtracted from the other side of the drive signal G*(A−x),as represented schematically by an oscillator 242 connected to the twodrive signal input lines to respective terminals V1, V2. The oscillator242 generates this fixed, small amplitude, high frequency (e.g., +/−1V,2 MHz) sensing signal, which is applied to the two capacitors of themicroactuator. When there is any imbalance between the two capacitors,meaning that the microactuator has moved away from the neutral position,a current with a frequency the same as the sensing signal frequency willappear at the middle point of the two capacitances to ground (shownschematically as AC current meter 243). The amplitude of the signal atthis particular frequency is proportional to the capacitance imbalance,which is again proportional to the position of the microactuator. Theamplitude of the current signal at the frequency of the added sensingsignal frequency is detected by amplitude demodulator 244 and the outputis the relative-position signal (RPS). The amplitude is zero when thetwo capacitances are equal. The use of a capacitance sensing circuit asa relative-position sensor for an electrostatic microactuator isdescribed in detail by M. T. White and T. Hirano, “Use of the RelativePosition Signal for Microactuators in Hard Disk Drives”, Proceedings ofthe American Control Conference, Denver, Colo., Jun. 4-6, 2003, pp.2535-2540.

The above-described method for relative-position sensing of anelectrostatic secondary actuator uses the force-generating elements forposition sensing, i.e., the sets of stationary and fixed electrodes. Ifthe secondary actuator is a piezoelectric actuator, the force-generatingpiezoelectric material, such as lead zirconium titanate (PZT), can beused for position sensing by monitoring the charge. In addition,relative position of a secondary can be sensed by integration ofstandard displacement using known sensors, such as potentiometers,strain gauges, encoders, capacitance probes, and piezoelectric material.

In the present invention the secondary-actuator failure detection andcalibration test comprises two measurements: a measurement of thesecondary actuator static characteristics and a measurement of thesecondary actuator dynamic characteristics. The static characteristicsmeasurement checks the secondary actuator movement amount per someconstant first test signal, and the dynamic characteristics measurementchecks the plant frequency response of the secondary actuator movementagainst a second test signal at one or more frequencies. The first andsecond test signals generated in the event of failure of the secondaryactuator are added to the drive signals, as described above for FIG. 10,and the RPS generated by relative-position sensor 240 in response tothese test signals is used in the measurements of the secondary actuatorstatic and dynamic characteristics.

1) Secondary Actuator Static Characteristics Measurement

FIG. 11 is a flow chart showing the details of the secondary actuatorstatic characteristics measurement (block 605 in FIG. 9). The secondaryactuator input is initially set as a neutral bias. A constant inputvoltage is then applied as a positive bias to the secondary actuator(block 605A) and the relative position is measured from the RPS (block605B) as described above. Next a constant input voltage is applied as anegative bias to the secondary actuator (block 605C) and the relativeposition is measured from the RPS (block 605D) as described above. Themeasurements (blocks 605B and 605D) are done multiple times and theaverage is calculated to get an accurate measurement. The secondaryactuator stroke is then calculated (block 605E) as the measured movementof secondary actuator for the known positive and negative voltageinputs. The calculated stroke is then compared with acceptable strokevalues at check point 610 (FIG. 9).

2) Secondary Actuator Dynamic Characteristics Measurement

FIG. 12 is a flow chart showing the details of the secondary actuatordynamic characteristics measurement (block 620 in FIG. 9). Thismeasurement is performed after the static characteristics measurement.The secondary actuator input is initially set as a neutral bias. Nextthe first frequency of the secondary actuator test signal is selectedand a sinusoidal input is made to the secondary actuator at the firstfrequency (block 620A). Generally the amplitude of the sinusoidal inputsignal is held constant during the entire measurement. Then the RPS isdetected for several samples and the secondary actuator relativeposition calculated (block 620B). FIG. 13 shows a typical test signal ata fixed frequency and a typical RPS response. (No units are listed onthe vertical axis because FIG. 13 is intended to merely show therepresentative shapes for a test signal and the RPS response.) Themagnitude of the RPS response is shown as the peak values, and the phasecan be roughly calculated as the difference between the zero crossingsof the test signal and the RPS response. However, for a more accuratecalculation of the gain and phase at the test signal frequency, adiscrete Fourier transform (DFT) is applied to the RPS values at theinput test signal frequency to calculate the gain and phase (block620C). Then the gain and phase for that input test frequency are storedin a table (block 620D). The next input frequency is selected for thetest signal (block 620E) and the process is returned to block 620A. Thesteps from blocks 620A to 620E are repeated until all desiredfrequencies are tested. For the convenience of the DFT calculation,multiples of the disk rotational frequency may be selected. After alldesired frequencies have been tested, the table is searched for themaximum gain value and its corresponding frequency (block 620F).

After the maximum gain value has been obtained, the check point (block625 in FIG. 9) is performed according to the details shown in FIG. 12.The maximum gain value is checked to determine if it is within anacceptable range of predetermined values (block 625A). For example, ifthe maximum gain is too low, e.g., not at least 20 db over the staticgain, this is an indication of actual failure of the microactuator(block 630 in FIG. 9). There are potential microactuator failure modesin which the motion is excessive, in which case the maximum gain valuemay be too high. If the maximum gain is not outside the range ofacceptable gain values, then the measured frequency at which the maximumgain occurs is compared to a predetermined acceptable range (block625B). For example, if the maximum gain occurs at more than 1 kHz fromthe known resonant frequency of the microactuator, this is an indicationof actual failure of the microactuator (block 630 in FIG. 9). However,if the frequency at which the maximum gain occurs is within acceptablelimits, then the controller parameters can be re-optimized (block 640 inFIG. 9). This re-optimization may take the form of a table lookup ofpre-calculated parameters or may be performed in real-time through theuse of an adaptive scheme. The controller may be adjusted to achievesuch features as desired bandwidth or stability margins, notching ofparticular frequencies such as the microactuator resonance, designingfor active damping of the microactuator resonance, or other performance,robustness, or stability metrics. However, if there is only a minimal orno substantial difference from the optimum values, then control can bereturned to block 505 (FIG. 9) without re-optimizing the controllerparameters.

FIG. 14 shows one example of the plant frequency response for a typicalmicro-electro-mechanical system (MEMS) type microactuator locatedbetween the suspension and the slider. The motion of this type ofmicroactuator is about 1 micrometer for a 30 V input at low frequency.So for the static characteristics test, a 0.5 micrometer movement wouldbe expected for a 15 V input. If the measured output was less than about0.2 micrometer or more than about 1 micrometer during the staticcharacteristics measurement, then this would likely indicate failure ofthe microactuator. In a server class disk drive, the spindle motorfrequency is between 167 Hz and 250 Hz, and the servo sector samplingfrequency is currently 50 kHz or above. So for the dynamiccharacteristics measurement, an acceptable appropriate frequency rangefor the series of input test signals would be from about 200 Hz to 10kHz. Thus as one example, the first sinusoidal test signal of FIG. 13could be at 167 Hz with subsequent test signals increased in 167 Hzincrements until the last test signal is reached at around 10 kHz. Thistype of microactuator also has a resonant frequency around 2 kHz, so ifthe maximum gain from the dynamic characteristics measurement is lessthan about 1 kHz or greater than about 3 kHz, then this would likelyindicated failure of the microactuator.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A method for operating a dual-stage actuator disk drive, the diskdrive including (a) a rotatable magnetic recording disk having aplurality of concentric data tracks containing servo positioninginformation; (b) a recording head movable across the disk, the headbeing capable of reading data and servo positioning information in thedata tracks; (c) a voice-coil-motor (VCM) for moving the head; (d) atest location where movement of the VCM is limited; (e) a microactuatorconnected to the VCM, the head being connected to the microactuator; (f)a relative-position sensor for detecting the position of themicroactuator relative to its neutral position; and (g) a servo controlprocessor responsive to servo information read by the head andcomprising (i) a dual-stage controller having a degraded-stability VCMcontroller with relatively high low-frequency gain and a microactuatorcontroller for simultaneously generating a VCM control signal and amicroactuator control signal, and (ii) a selectable single-stagecontroller for generating only a VCM control signal, theprocessor-implemented method comprising: detecting a potential failureof the microactuator; generating a location signal to the VCM to movethe VCM to the test location and selecting the single-stage controllerif a potential microactuator failure is detected; generating a firsttest signal to the microactuator; receiving a relative-position signal(RPS) from the relative-position sensor in response to the first testsignal; calculating the stroke of the microactuator from the first testsignal and the RPS; and posting a microactuator failure signal if thecalculated stroke is outside a predetermined stroke range.
 2. The methodof claim 1 further comprising, if the calculated stroke is within thepredetermined range, generating a series of second test signals to themicroactuator, each of the second test signals in the series being at afrequency different from the other second test signals in the series;receiving a series of second relative-position signals in response tothe series of second test signals; recording the gains and correspondingfrequencies from each of the second relative-position signals; andposting a microactuator failure signal if the maximum recorded gain isoutside a predetermined range.
 3. The method of claim 2 furthercomprising, if the maximum recorded gain is within a predeterminedrange, posting a microactuator failure signal if the maximum recordedgain is at a recorded frequency outside a predetermined frequency range.4. The method of claim 2 further comprising, if the maximum recordedgain is at a recorded frequency within said predetermined frequencyrange, modifying the parameters of the controller from the recordedgains and corresponding frequencies.
 5. The method of claim 4 furthercomprising terminating subsequent modification of the controllerparameters if the controller parameters have been modified more than apredetermined number of times within a predetermined time period.
 6. Themethod of claim 1 further comprising, if the calculated stroke is withinthe predetermined range, generating a series of second test signals tothe microactuator, each of the second test signals in the series beingat a frequency different from the other second test signals in theseries; receiving a series of second relative-position signals inresponse to the series of second test signals; and recording the phasesand corresponding frequencies from each of the second relative-positionsignals.
 7. The method of claim 1 wherein the servo control processoralso includes a model of the dynamic response of the VCM and a model ofthe dynamic response of the microactuator, and wherein theprocessor-implemented method further comprises: providing a modeledhead-position output when the control signals from the dual-stagecontroller are input to the models; and wherein detecting a potentialfailure of the microactuator comprises determining if the differencebetween the modeled head-position output and the measured head positionfrom the servo information read by the head is greater than apredetermined value.
 8. The method of claim 1 wherein the servo controlprocessor also includes a model of the dynamic response of themicroactuator, and wherein the processor-implemented method furthercomprises: providing a modeled output of the position of themicroactuator relative to the VCM when the microactuator control signalfrom the dual-stage controller is input to the microactuator model; andwherein detecting a potential failure of the microactuator comprisesdetermining if the difference between the modeled microactuator relativeposition and the measured relative microactuator position from therelative-position sensor is greater than a predetermined value.
 9. Themethod of claim 1 wherein the microactuator is an electrostaticmicroactuator having a first set of movable and fixed electrodes and asecond set of movable and fixed electrodes, and wherein receiving a RPScomprises measuring the change in capacitance across each set ofelectrodes in response to the test signal.
 10. The method of claim 1wherein the test location is a VCM crash stop and wherein generating alocation signal to the VCM comprises biasing the VCM against the crashstop.
 11. The method of claim 1 wherein the test location is aload/unload ramp and wherein generating a location signal to the VCMcomprises biasing the VCM against the ramp.
 12. A method for operating adual-stage actuator disk drive, the disk drive including (a) a rotatablemagnetic recording disk having a plurality of concentric data trackscontaining servo positioning information; (b) a recording head movableacross the disk, the head being capable of reading data and servopositioning information in the data tracks; (c) a primary actuator formoving the head; (d) a crash stop where movement of the primary actuatoris limited; (e) a secondary actuator connected to the primary actuator,the head being connected to the secondary actuator; (f) arelative-position sensor for detecting the position of the secondaryactuator relative to its neutral position; and (g) a servo controlprocessor responsive to servo information read by the head andcomprising a dual-stage controller for simultaneously generating aprimary actuator control signal and a secondary actuator control signal,the processor-implemented method comprising: generating a crash stopsignal to the primary actuator to move the primary actuator to the crashstop; generating a first test signal to the secondary actuator;receiving a relative-position signal (RPS) from the relative-positionsensor in response to the first test signal; calculating the stroke ofthe secondary actuator from the first test signal and the RPS; andposting a secondary actuator failure signal if the calculated stroke isoutside a predetermined stroke range.
 13. The method of claim 12 whereinthe primary actuator is a voice-coil-motor (VCM).
 14. The method ofclaim 12 wherein the secondary actuator is an electrostaticmicroactuator having a first set of movable and fixed electrodes and asecond set of movable and fixed electrodes, and wherein receiving a RPScomprises measuring the change in capacitance across each set ofelectrodes in response to the test signal.
 15. The method of claim 12wherein the dual-stage controller has a degraded-stabilityprimary-actuator controller with relatively high low-frequency gain anda secondary-actuator controller for simultaneously generating aprimary-actuator control signal and a secondary-actuator control signal,and (ii) a selectable single-stage controller for generating only aprimary-actuator control signal.